1. Field of the Invention
The present invention relates generally to internal combustion engines for automotive vehicles and, more particularly, to a method of controlling idle speed for an internal combustion engine in an automotive vehicle.
2. Description of the Related Art
Controlling idle speed of an internal combustion engine in an automotive vehicle poses some very common problems which include limiting idle roughness, controlling vibration, and controlling long-term small oscillations in the idle speed. These common problems have been a challenge in the automotive vehicle for a long time. To overcome these common problems, the engine has been provided with an idle speed control system. A typical idle speed control system or Automatic Idle Speed (AIS) control system uses a feedback method through an AIS stepper motor to maintain a consistent idle speed for the engine.
Because of resolution limits and delay effects (such as plenum filling, stroke and fuel flow delay, etc.), the AIS control system only provides a "coarse" adjustment in the control of the idle speed for the engine. During idle of the engine, the natural idle speed (i.e., idle speed without spark stabilization) is usually away from a targeted idle speed. When the AIS stepper motor moves, it can contribute to long-term oscillation in the idle speed and, combined with spark stabilization, may deteriorate the idle speed control quality of the engine if there is too much spark change.